This invention relates generally to atomic or ionic devices and, more particularly, to atomic beam devices employing optical pumping to increase the populations of selected states. The advantages of such population enhancement are illustrated by consideration of a cesium clock which employs an atomic beam of cesium.
In cesium, the nucleus has spin 7/2 which combines with the spin 1/2 of the valence electron to produce ground energy level (i.e., 6.sup.2 S.sub.1/2) states of total angular momentum F=3 and F=4. These states have a difference in energy that can be used to produce a very accurate clock signal. The energy of the sublevels of these hyperfine states as a function of applied magnetic field is shown in FIG. 1. For zero applied field the states separate into two hyperfine states of seven F=3 sublevels and nine F=4 sublevels. These states differ in energy by the hyperfine energy difference. For a strong applied magnetic field the states separate into two groups of eight sublevels: (1) a group of F=4 sublevels with m.sub.F =-3, -2, . . . , +4 which increase in energy with increase in applied field and (2) a group of sublevels with F=3 and with F=4 and m.sub.F =-4 which decrease in energy with increase in applied field.
In the cesium tube shown in FIG. 2 the energy difference between the atomic states of cesium is used to produce a very accurate and stable clock frequency (see Hyatt, et al., "A High Performance Beam Tube for Cesium Beam Frequency Standards", Hewlett-Packard Journal, September 1973, p. 14-24). An oven and collimator are employed to produce a beam of cesium atoms which, because of the small energy difference between the F=3 and F=4 states, are roughly evenly distributed among the sixteen 6.sup.2 S.sub.1/2 sublevels. The beam passes first through a strong inhomogeneous magnetic "A" field. It then goes through a microwave cavity in a weak homogeneous magnetic "C" field region. Next it goes through a second strong inhomogeneous magnetic field, the "B" field and finally goes to the detector. The oven, "A" magnet, and "B" magnet are so arranged and fitted with beam stops that only atoms originally in the F=3, all sublevels, and the F=4 m.sub.F =-4 sublevel entering the "A" magnet can get through the "B" magnet gap. The detector is placed so that only atoms in the F=4 m.sub.F =-3, -2, -1, 0, 1, 2, 3, 4 states entering the "B" magnet will be detected. Therefore, in the absence of RF excitation of the microwave cavity, no atoms will be detected.
The weak homogeneous magnetic "C" field is about 0.09 Gauss to produce a weak field splitting of the energies of the group of F=3 sublevels and of the group of F=4 sublevels (see FIG. 1). An rf source provides an rf field in the cavity to induce transitions from the F=3 m.sub.F =0 state to the F=4 m.sub.F =0 state. The number of transitions is maximized when the rf frequency equals the transition frequency corresponding to the energy difference between the 6.sup.2 S.sub.1/2 F=3 m.sub.F =0 state and the 6.sup.2 S.sub.1/2 F=4 m.sub.F =0 state. These atoms will now be deflected by the "B" magnet into the detector which provides an amplified signal proportional to the number of atoms striking it per second. This signal is used to regulate a voltage controlled crystal oscillator (VCXO) to produce an output signal of precisely regulated frequency (in the cesium clock shown in FIG. 2 this frequency is 5 MHz). The 5 MHz signal is supplied to an output for use as a clock signal. The 5 MHz signal is also supplied to an rf source to produce a frequency modulated rf field of carrier frequency equal to the transition frequency. In the cesium clock of FIG. 2 the modulation frequency is 137 Hz and the carrier frequency is about 9192.631774 MHz.
Because of the frequency modulation, the detector signal has a 137 Hz component when the rf carrier frequency strays from the transition frequency. The detector signal is applied to a synchronous detector to produce a directional error signal proportional to the 137 Hz component of the detector signal. The error signal is applied to an integrator which provides an integrated error signal used to control the VCXO to produce a 5 MHz signal.
For high gain in the feedback loop from the detector to the VCXO, the accuracy of the 5 MHz output signal becomes that of the cesium beam tube. The fractional amount of noise introduced into the 5 MHz signal by the cesium tube decreases with increased beam flux to the detector. Because essentially only those atoms reach the detector which leave the oven in the 6.sup.2 S.sub.1/2 F=3 m.sub.F =0 state and are excited to the 6.sup.2 S.sub.1/2 F=4 m.sub.F =0 state, fifteen-sixteenths of the beam leaving the oven is not utilized. A significant improvement in signal noise can thus be achieved by a scheme which transfers a major fraction of the atoms leaving the oven into the 6.sup.2 S.sub.1/2 F=3 m.sub.F =0 (or F=4 m.sub.F =0) state before passing through the microwave cavity.